This invention relates generally to control devices for mixing water of different temperatures and more particularly it relates to such a mixture control device for use in connection with a warm water installation having a relatively low supply temperature.
In view of the shortage of fossil fuels, there are increasingly employed heat generators employing solar energy, heat pumps and long-distance heating for raising the temperature of heating water and of service water.
Heating energy derived from sun and the heating energy derived from heat pumps are to be generated, the former for technological reasons and the latter for thermodynamic reasons, at as low temperatures as possible, such as for example at 40.degree. C.
Also, in the case of the long-distance heating, measures are being taken to lower the return temperature from conventional 70.degree. C. to a temperature below 40.degree. C. in order to eliminate the loss in thermal efficiency of a power plant resulting due to the so-called "heat-power coupling" at a high return temperature.
In the case of the service water supply this lower temperature is sufficient for all applications having high water and energy consumption such as for example washing, bathing and showering. Only for the rinsing of dishes and for the additional heating of cooled-down bathing water, higher water temperatures are necessary. For this reason contemporary warm water reservoirs having the relatively low temperature are equipped with flow heaters which heat up the service water from about 40.degree. C. to about 70.degree. C. The additional heating can be effected with advantage only electrically inasmuch as it is the primary purpose of such installations to eliminate, at sufficiently high ambient temperatures, the use of auxiliary heating units operating on fossil fuel.
The electrical after-heating of service water which has been already sufficiently raised in temperature and the subsequent cooling down thereof to a service temperature by mixing the after-heated water with a cold water contradicts however to the thermodynamical principle according to which the heat should be generated at such a temperature at which it is used.